Commando: the source

Current truth for this game. The workflow lives in kit/skills/; how this understanding developed lives in agent-history.md. Every fact names the routine or table it comes from. Unless marked live, a fact comes from reading the code in the snapshot named in orientation.md (work/entry.vsf, the machine stopped at the game's entry, $0850).

Build

The disk is REMEMBER's crack of Elite's 1985 game, released in June 1997 as "Commando +5HD" (CSDb release 40672), with the directory header once upon a time and two files: COMMANDO+5HI/REM, the packed program, and COMMANDO HI /REM, the saved high-score table. The crack shows instructions, offers five trainer options and saves the high scores to disk. Everything it added is named below ("REMEMBER's changes"); the rest is the 1985 game, whose title screen credits Elite, Chris Butler (programming), Rory Green and Chris Harvey (graphics), Rob Hubbard (sound) and Japan Capsule Computers UK ($1143).

The 1985 release it is compared with below is Elite's PAL disk in the C64 Preservation Project's 10th Anniversary Collection (commando_elite_1985_pal, a G64), stopped as it hands over to the game at $0850 (work/orig1985/entry1985.vsf).

Memory layout

The VIC-II looks at bank 3 ($C000-$FFFF, CIA 2 port A, $358F), so the character sets at $D000 and $D800 lie in the RAM under the I/O area and the screen and sprites under the KERNAL ROM. The processor port reads $36 in play: BASIC out, KERNAL and I/O in. Two routines change it: $0A57 clears CHAREN to copy the name-entry gunsight into $D040, and $0AFA clears HIRAM to read the screen at $E000.

The game is one load. All three maps, the character sets and the sprites are in memory from the start; nothing is read from disk during play.

Timing

The machine is a PAL C64. The raster interrupt at $4119 counts a frame at $040B, and the main loop ($08CB) waits for that count to change ($400C), so the game runs one pass a frame and a pass that overruns the frame waits for the next count. Live: 497 passes of the loop in 500 frames walking up the first area; 60 of them, one for each map row the screen moved, skipped the game logic, because the loop does the row's redraw instead ($08CB). Music and sound effects run from the interrupt at $416A, every frame.

Controls

• Joystick in port 2: eight directions and fire. Fire alone starts a game from the title ($0F48).
• Space throws a grenade ($38A5). The game reads the keyboard row at $DC01 itself, because it stops CIA 1's timer and with it the KERNAL's keyboard scan ($40E8). In this copy the read goes through REMEMBER's hook at $5F6A.
• RESTORE does nothing: the NMI vector points at an RTI ($0A4D).

Text

The game writes with its own alphabet in the text character set: space $20, digits $21-$2A, letters A-Z $5B-$74, the grenade and equals signs of the score line $3C and $3F, and the name entry's marks, rub-out and END $75-$78. Its strings: the credits ($1143), the score line ($40C0), the four lines of the message between areas ($12B8), their ordinals ($1306), the ranks of the high-score table ($0ECE) and the letter grid of the name entry ($0D09). REMEMBER's texts are in the standard screen codes ($4660, $46D8, $4800, $5F80-$5FFF, $7F60).

Graphics

• Screen. Multicolour characters, 24 rows ($4049): 21 rows of playfield copied from the area's map ($3F75) and the score line under them in the text character set. Background brown, the shared colours black and grey; the playfield's own colour is green, blue in area 1 ($40BC).
• Scrolling. The map scrolls down a pixel a frame while the player pushes up near the top of his zone ($3A8C, $0404). Every eighth step the map moves a character row ($0403) and the whole playfield is redrawn from the map.
• Areas. The area counter's low two bits pick the map, the character set ($D018 = area x 2, $1427), the spawn list and the terrain table. Each map is 200 rows; a new area starts near the bottom and the gate or fortress is at row 0.
• Terrain. Each area has a class for each of its 256 characters ($178B, $188B, $1A8B): bit 0 a wall, which stops the player, turns soldiers aside and stops bullets; bit 1 scenery the sprites pass behind; bit 2 water, a pit or a trench, which the player falls into.
• Sprites. Sixteen slots share the eight hardware sprites. On each pass of the game logic ($08F7) $3F06 sorts the slots by y (a Shell sort, gaps 7, 3 and 1, over $4B-$5A), and a chain of raster interrupts places them: $41A7 at line $1E puts the first eight on sprites 7-0, $4266 reuses sprites 7-4 for sorted slots 8-11 below the fourth, and $436B reuses sprites 3-0 for slots 12-15. $4119 at line $D5 blanks the screen for the score line and $416A at line $DE shows it.
• Map changes. Grenades that destroy a bunker gun put a ruined bunker into area 1's map ($14B5, $37B1), and every life puts the three whole bunkers back ($3DE0). The gate at the top of areas 0 and 1 is drawn shut at each life's start and open when its row comes up ($15BC).

Mechanics

Slots. Slot 0 is the player, 1-3 his bullets, 4 his grenade (an empty slot 4 can carry a bullet too), and 5-15 the enemies. Their tables run from $040D (x-high), $041D (x), $042D (y), $043D (behind scenery), $044D (colour), $045D (sprite block), $046D and $047D (speeds) and $048D (type or behaviour), sixteen bytes each; the enemies also have a parameter ($04A1), a facing ($04AC) and a timer ($04B7).

The player. Up moves him a pixel a frame until he reaches y $A4, and above that scrolls the map instead; at the top of the map (row 0) nothing scrolls and he walks on up to y $6E. Down goes to y $C1, and left and right two pixels a frame between x $17 and $141 ($3A8C). His facing turns a sixteenth of a circle a frame towards the stick ($35B0), and his bullets fly in the facing, so a reversal takes a few frames to come round.

The gun. Each press of fire gives two bullets ($39C3): a counter ($04DF) set to $FF while fire is up counts, on each pass of the game logic ($08F7), once for each empty shot slot, and a bullet goes when it reaches 0 and again at 8, where it stops: two frames after the first with all four shot slots empty, three with three. Live: four presses held for 40 frames each made two bullets each. A bullet lives 15 frames, at up to eight pixels a frame across and six up or down ($35D9, $35E9), and stops at walls ($3917).

Grenades. A new game gives five grenades and five lives ($0883), and every life tops the grenades up to five ($3DE0). Space throws one straight up the screen; it flies 40 frames, then blasts for 20 frames, killing anything a grenade can hurt within 18 pixels across and 22 up or down ($365C, $371E). A box of grenades adds three and 1,000 points ($2A5A); the count is two BCD digits and the add keeps no carry, so 98 plus a box makes 01. Live: poked to 98 under a box, the count read 01. The pickup's effect requests return with X = 0 ($5F56), and when the 1,000 points carry into a new 10,000 the extra life's request comes before the box's slot is freed ($3570): enemy slot 0 is freed instead, the enemy loop goes on from enemy 1 ($2495), and a box in any later slot is picked up again, for six grenades, 2,000 points and the life. Live: a box in enemy slot 2 at 9,500 points made 11 grenades from 5, 11,500 points and a life, and the rifle soldier in enemy slot 0 vanished; at 1,000 points the same box paid once.

What hurts what ($2526). Bullets and grenades kill the walking soldiers. Only a grenade kills a soldier lying in wait, in a trench or foxhole, a bunker's gun, the lorry, the sniper or the mortar man. Only a bullet counts against the guards. Prisoners, vehicles, jeeps and doors are never hit. The player dies on touching an enemy whose behaviour is marked in $1074: soldiers on the move, enemy bullets, bombs and the bunker guns' shells, explosions, the leading half of a crossing vehicle, the lorry and the jeep; not guards, soldiers in cover, the stopped motorcycle, or grenades and mortar bombs in flight. $100F compares positions: the same x-high half, the enemy's x from 3 pixels left of the player's to 4 right, its y from 7 above to 8 below. Walking into water or a pit is the other death ($3C10).

Enemies. Each of the eleven enemy slots runs one of 41 behaviours ($24D4). They come from two places:

• the area's spawn list, an entry for a map row, started when that row reaches the top of the screen ($1B8B), by one of 28 handlers ($1BE8);
• an empty slot, which now and then starts a rifle soldier from the top of the screen or a grenade thrower from a side ($2D63).

Three kinds have handlers but no entry in the lists in use: the doors that let soldiers out (handlers 23 and 24), the mortar man (26) and one grenade thrower (10). The doors and the mortar man are in the unused fourth list ($1D42); handler 10 is in none.

The fourth list has 25 entries, the last the end of the area (handler 27): six doors of handler 24 and three of 23, a box (7), a motorcycle (4), a grenade thrower (12), two walking soldiers (8, 9), two lorries (20), three vehicles crossing (22) and one more (19), a trench soldier (3), the mortar man (26), the gate opening (11) and a green soldier (13). Its first rows, 198 and 196, are below the start of every area (rows 175 and 166, $3EBC). Drawn over each map at the rows and x its entries give, with each handler's starting y (doors $28, $1E55; the box $26, $2111; the trench soldier and the mortar man $21, $221E): eight of the nine doors stand in open ground on all three maps, and the ninth (row 157, x 56) on the left gate of area 3's first wall. On area 3's map the trench soldier and the mortar man (row 9) stand in the fortress's two windows, a row above the two soldiers area 3's list puts there (row 10, $1DA7). Its two walking soldiers (both at row 94, with x 80 and x-high 99) have the x and x-high of area 1's two (rows 97 and 94), and its grenade thrower is at area 1's row 104 ($1CC1).

A rifle soldier fires only on frames where his timer AND the fire mask $0504 is 0, and only when he already faces the player's direction ($33B2). The fire mask is set at each life from the area counter's low three bits ($3EB4): a shot every 64 frames at most in the first area, 32 in the second and 16 in the third. Each gate soldier sent out sets it back to 64 for the rest of the life ($2CD1).

The direction is one of eight, from a box 30 pixels to either side of the soldier and 20 above and below ($33C7; $310A turns him to face it, a rifle soldier on frames where the frame counter's low five bits are 0, $3219). Inside the box he neither turns nor fires. The x test adds 30 to the low byte of the soldier's x and drops the carry, so a soldier at x 226-255 (x-high clear) takes a player to his left, past the wrapped sum, for one on his right: he faces right and fires right. Live: a rifle soldier held at x $E8 with the player held level at x $64 faced right (heading 4) and fired every bullet at +2 pixels a frame; at x $C8 he faced left (12) and fired at -2.

The gate. At the top of each map twenty soldiers come out of the gate or the fortress: each free enemy slot has one chance in 128 a frame of sending one while any remain ($2CBB, $04EF). The area is won when all twenty have come and none was counted alive that frame ($3A8C, $04F4, counted afresh each frame by the gate soldiers' behaviour, $31D2). A soldier entering area 1 from a side is not counted on his first frame ($2D08), so if the twentieth comes that way with no other gate soldier alive, the area ends as he appears. The gate opens ($3A2E), the player walks himself to x $AF and up through it ($3A3B), and at y $5A the area ends with 1,400 points ($0923).

Areas. The area counter ($04F3) skips every value whose low two bits are 2 ($0939), so the areas played are 0, 1 and 3, and area 2's character set ($D000) only ever shows text: the title, the name entry and the messages set the counter to 2 while they run ($0F53, $0C9E, $1270). Area 2's entries in the per-area tables are area 1's map ($3EB0) and restart rows ($3ED0), area 3's spawn list ($1485-$14A4) and terrain table ($14A5), and a copy of area 3's ordinal ($1306). After area 3 the ending runs ($3CB2, eleven flickering red bursts across the fortress for 255 frames), the message says CONGRATULATIONS and MISSION COMPLETE ($1222), and play starts again at area 0 with the counter at 4. The fire masks for counters 4, 5 and 7 are all 16 frames, and counter 8 begins again at 64, so the loops alternate between the first loop's rising rates and a loop fired at full rate. Live: the counter went 0, 1, 3, 4 with fire masks $3F, $1F, $0F, $0F, and the messages read BROKE THE 1ST AREA, BROKE THE 2ND AREA and CONGRATULATIONS.

Death and restart. A hit shows the player struck and then lying, 79 frames; a fall shows him sinking, 65 frames ($0953). A life restarts at the first checkpoint at or below the row the screen had reached ($3DE0): rows 19, 61, 97 and 131 in every area, and 175 in area 0 or 166 in areas 1 and 3 ($3EBC). A new area parks the map at row 165, so play itself starts at that last checkpoint. Live: areas began at rows $AF, $A6, $A6 and $AF. A restart brings back everything in the spawn list within 21 rows of the top ($109B) and sets the gate's twenty soldiers again.

Score and lives. The score is two BCD bytes counting hundreds ($04F8, $04F9), shown as four digits and two fixed zeros ($136F). Each carry out of the low byte, every 10,000 points, gives a life ($134F). Points per enemy are in $254F: 200 for a foxhole, 300 for most soldiers, 500 for a trench soldier, bomb thrower, sniper or mortar man, 1,000 for a guard or either half of a lorry; a bunker gun's 1,000 comes from the grenade's own path ($37B1). The green soldier leaves a sprite reading 2000 but adds 1,400 ($23E7). The lives ($0500) are a binary count printed as two hex digits ($13F1), so from ten the second digit is not a digit. Live: 9,900 plus an area's 1,400 made 11,300 and five lives from four; 999,900 plus 1,400 made 001,300; ten lives showed as 0 and a block graphic.

High scores. At game over the score is ranked into the eight-entry table ($09A4). A place in it starts the name entry ($0C88): the player stands at the foot of a grid of letters and shoots them with a gunsight he steers, up to eight letters, with rub-out and END. The table is shown until fire, and in this copy REMEMBER's saver writes it to disk and restarts the game ($0A4A, $44E0).

Data tables

Sound

Rob Hubbard's driver sits at $5000 behind a row of jumps ($5000 start tune, $500F request effect, $5012 play a frame). Three tunes, listed at $56FF:

Notes come from 45 patterns, each voice's track a list of pattern numbers ($5711). There are thirteen instruments of eight bytes ($5591) with drum, vibrato, pulse-sweep and octave-trill effects ($519B-$5390). The note table holds 96 frequencies ($5428). Sixteen sound effects are defined ($55F9) and eight are used: grenade box, grenade thrown, bunker destroyed, player hit or fallen, the jeep, the last area won, a letter shot and an extra life. An effect takes the SID's first two voices while it sounds; the third voice always plays the music ($538F).

I/O registers

Never touched: the sprite collision registers $D01E and $D01F (the game compares positions itself), the light pen, the SID's filter ($D415-$D417) and its read-back registers, and CIA 2 apart from the VIC bank.

REMEMBER's changes

• Added code: the saver ($44E0), the menu and trainer ($4531), the table loader ($463D), the instructions viewer and its two pages of text ($4710, $4800), the key hook ($5F6A) and their texts.
• Hooks into the game: the jump after the name entry goes to the saver ($0A4A), and the space-bar read at $38BE calls $5F6A.
• Removed: the 1985 release's reset trap at $085C-$0868, replaced by LDA #$1B / STA $D011 and eight NOPs, and the nine bytes it copied ($0A4E), zeroed.
• The bug fix: the high-score walk starts at the eighth entry ($09A5, $0A0D = 7) where the 1985 release starts at the seventh (6).
• The trainer ($4531) patches the game: unlimited lives turns the DEC at $0996 and the INC at $1363 into LDAs; unlimited grenades zeroes the box's 3 at $2A8F and the throw's 1 at $3909; no falling fills $3C10-$3C5E with NOPs; no collisions fills $1056-$106D with NOPs; the level skip turns the RTS at $5F6D into a CMP, so C= ends the area. Choosing the trainer also points the save jump back at the title, so a trained game saves no scores.
• The high scores on disk are loaded over $0EED-$0F47 before the game starts; the 91 bytes of COMMANDO HI /REM are that range.

The 1985 release

Both copies stopped at the hand-over to the game ($0850) and compared byte for byte (work/scripts/diff1985.py): of the game's 19,450 bytes, $0850-$44DF and $5000-$5F69, 29 differ. Every byte that differs from $0850 up, 4,298 of them, is in this table:

The maps, character sets and sprites are the same apart from $C71E-$C71F, the last two rows of glyph $E3 in area 0's character set, which area 0's map never uses. The screen, the gunsight's sprite block ($D040) and the blank sprite block are written before they are shown ($3F75, $0A57, $3E93). The high-score table at $0EED-$0F47 is the same in both: REMEMBER's file on this disk holds the 1985 release's own starting table.

The bug fix. $09A4 walks the table from entry X up. With 6, a score that does not beat entry 6 (the seventh) stops the walk at once and is stored in entry 7 (the eighth), over its score and under its name, and the test at $0A0C (CPX #$06) sees that the walk never moved and goes back to the title without a name entry. With 7, such a score goes into the spare ninth entry. test_table.js runs the game's walk with both values over 820,000 cases against the page's port (index.html, the commando-table block).

The reset trap. $085C copies $0A4E-$0A56 to $8000-$8008: $0850 as the cold and warm starts, then CBM80 with bit 7 set, so the KERNAL's reset jumps to $0850. RESTORE does nothing in either copy, because $0850 points the NMI at an RTI ($0A4D). After a reset the processor port's direction register $00 is 0, the game's writes to $01 change nothing, and the gunsight copy ($0A57) writes its 64 bytes into the video chip's registers: $D011 gets $20 (from $0A90) and the display goes off. Nothing sets the display bit again: the game's other writes to $D011 ($404E, $40F2, $411F, $4133, $4152, $4171, $418B, $41B7, $424E, $4353 and $4451) each keep the bit as they read it. The reset also leaves CIA 1's port A all inputs ($DC02 = 0), so it reads $EF with fire, and the title's test for fire alone ($0FB2, CMP #$6F) never passes: the restarted game cycles its title and attract views and cannot be started. The processor port's lines, all inputs, read high, so the BASIC ROM stays in view over area 3's map at $A000, and the attract views of area 3 are drawn from it. The copy also overwrites the first nine bytes of area 1's map, row 0's columns 0-8.

The original's music routines. $5F6A points $0314 at $5F81, which acknowledges a raster interrupt, sets the next at line $80, writes $1B to $D011, plays the music ($5012) and goes on to $EA31. $5F96 puts $EA31 back, silences the SID ($D418 = 0), restarts CIA 1's timer and turns the raster interrupt off. No word in memory points at either, and neither ran from the boot to the hand-over (work/scripts/boot1985_music.py).

The listing's error rate

Two random samples, each checked against the bytes by an agent that wrote none of the comments. The first (seed 20261010) drew 60 of the 837 comments written by hand and 10 of the 24 on map data, and found 12 wrong, 17.1 % (Wilson 95 % interval 10.1-27.6 %): a frame count given for a random chance and for a pass of the game logic, a caller list too broad, one sprite block taken for another, $D018's low nibble given as the register, two counts off by one frame, and four map descriptions with a row or a terrain class wrong. Those twelve were corrected, and four further agents audited every comment in four disjoint ranges and corrected 137 of 897 (15.3 %), with the same kinds of error. The second sample (seed 20261011), drawn after the audit from 868 comments by hand and 29 on map data, took 60 and 10 and found 1 wrong, 1.4 % (Wilson 95 % interval 0.3-7.7 %; 1.6 % weighted by the strata's sizes): $2495 said the enemies run every frame, where they run on every pass of the game logic, which misses the frame on which the map moves a whole row. That comment and fifteen others making the same claim about routines the logic pass calls were corrected.

Live tests

All from work/entry.vsf, the game's entry, in VICE with the joystick on port 2 through the emulator's own control.

• After fire on the title: five lives, five grenades, area 0, twenty gate soldiers to come, fire mask $3F.
• Four presses of fire, each held 40 frames and released for 40: two bullets each (a checkpoint on the bullet's set-up at $3A13). Stepping through $39E8 showed the counter reach 8 and stop.
• With REMEMBER's own no-collision and unlimited-lives pokes, setting the gate soldiers to come ($04EF) to 0 three times: the counter went 0, 1, 3, 4; each area added 1,400 points; the fire masks were $3F, $1F, $0F, $0F; the map started at rows $AF, $A6, $A6, $AF; the screen showed BROKE THE 1ST AREA, BROKE THE 2ND AREA, then CONGRATULATIONS.
• Score 9,900 and an area won: 11,300 and a life more. Score 999,900 and an area won: 1,300 and a life more.
• Grenades poked to 98 and a box put under the player in slot 5: 01 grenades, 1,000 points, the box gone.
• A box in enemy slot 2 under the player and a rifle soldier in enemy slot 0 (work/scripts/boxbug.py): at 9,500 points, 11,500, grenades 5 to 11, a life, and both slots free; at 1,000 points, 2,000 and 8 grenades, the soldier still there.
• Lives poked to eleven and one lost: the count read 0 and glyph $2B.
• 500 frames walking up: 497 passes of the main loop, 437 of them running the game logic, against 498 passes of the bottom raster interrupt.
• With REMEMBER's unlimited-lives patch, lives held at five through the area tests' deaths (work/scripts/live.py unlimited).
• The attract mode, stopped as each view was set up ($0FD8): seven views of 255 frames (area 0 rows 83 and 124, area 1 rows 58 and 1, area 3 rows 169, 110 and 54), then an eighth set up from one byte past each table (row $A0, counter $53) and drawn over at once by the title, which showed 60 frames later (work/scripts/attract.py).
• A rifle soldier and the player held in place every frame (work/scripts/aimbug.py): at soldier x $E8, player x $64, level, the soldier faced right and his bullets moved at +2; at x $C8, left and -2.
• The 1985 release, one life and a score of 2,500 at the game over (work/scripts/live1985.py table orig 0025): no name entry ($0C88 did not run), and the eighth entry read DILK 002500. This copy, the same: the name entry ran, the eighth entry took 002500 and DILK 002000 went to the ninth. At 0, the 1985 release's eighth entry read DILK 000000 with no name entry, and this copy's table did not change. At 250,000 both ran the name entry and put the score first.
• At $0869, the 1985 release's $8000-$8008 read 50 08 50 08 C3 C2 CD 38 30, and this copy's nine $CC (live1985.py trap).
• A soft reset at the title (work/scripts/reset1985.py): the 1985 release stopped at $0850 with $00 = 0, and 300 frames on the processor read the BASIC ROM at $A000 (94 E3 7B E3). A hard reset, which clears the RAM, and either reset of this copy went to BASIC.
• After that reset, the first write to $D011 was the gunsight copy's at $0A6A, with $20, and $D011 read $07 300 frames on (work/scripts/reset_d011.py, reset_regs.py).
• After that reset, six presses of fire 100 frames apart never reached the game's start ($0FEB, $08B8): the attract views went on changing, and $DC02 read 0. With $DC02 = $FF and $DC00 = $7F written first, the first press started a game with five lives (work/scripts/reset_fire.py, reset_fire.py cia).
• The attract view of area 3 at row 169, with and without that reset and with $D011 = $1B written just after the gunsight copy ($0A70): the map drawn as usual without it, scrambled characters with it, and the processor reading 94 E3 7B E3 (BASIC) at $A000 against 35 30 30 30 (work/scripts/attract1985.py; the two pictures are in reference/).
• Area 1 walked to its top with REMEMBER's pokes (work/scripts/walk1985.py): the screen's row 0 began 50 08 50 08 C3 C2 CD 38 30 in the 1985 release and CC in this copy, and the two screenshots differed only in two pixel lines at the top.
• REMEMBER's unlimited-grenades patch ($2A8F and $3909 = 0): four throws ($3903 ran four times) and a box left the count at 5; the same run without the patch went 5 to 1, and the box added 3 (work/scripts/live.py grenades).

Cheats

Pokes that change the game within its own parameters. Each one names the variable it changes and whether it has been tested live. Untested pokes are labelled as candidates.